Several recent studies have demonstrated that ionizing radiation activates a metabolic process resulting in the transient generation of reactive oxygen (ROS). The underlying mechanism for this amplification and signaling mechanism of cytoplasmic ionization events appears to involve a Ca2+ dependent signal propagated from one mitochondria to another within a cell. Temporally coincident is the activation of a constitutive, Ca2+ dependent nitric oxide synthase (NOS*). It is proposed that the radiation-induced activation of NOS is a mechanism that (1) limits the amount of cytotoxic, mutagenic ROS (02-/H202) formed after radiation; and (2) activates signal transduction pathways involved in cell growth regulation. Because of its unique physiochemical properties, the enhanced NO. generation after irradiation also provides a mechanism for the intracellular and intercellular propagation within a tissue of a signal generated by an ionization event that has occurred in a single cell. To test this proposal, there are two specific aims: Determine whether modulation of NOS activity by molecular and pharmacological means enhances radiation-induced cell killing and/or mutagenesis that in turn can be reduced by over expression of superoxide dismutase (SOD) or treating cells with SOD mimetics. Determine whether manipulation of NOS activity modulates radiation-induced ROS generation measured as 02 Test (a) whether manipulation of the NO-/sGC/PKG signal transduction pathway alters cellular radiosensitivity; (b) the relationships between radiation-stimulated sGC/PKG and downstream consequences including transcriptional activation of c-fos and p21 Waf1/Cip1 and activation of members of the MAPK family and (c) whether radiation-stimulated transient S-nitrosylation and inhibition of cytosolic and nuclear protein tyrosine phosphatases (PIP) are involved in cellular responses to radiation (SHP-2, SHP-1 and cdc25).